1. Field of the Invention
The present invention relates to an image reader for a scanner, copier or the like provided with a reading position alignment function.
2. Description of the Related Art
In recent years, a flat bed type image scanner device capable of inputting an image in a simple manner has come into wide use. The coordination of the image scanner device and a host device during transfer exists as a problem associated with the improvement in the precision of image reading and the improvement in the image reading speed. Especially, in the case where the image reading speed of the image scanner device is higher than the data transfer speed of the host device, it is necessary to coordinate these devices by use of any means.
In such a case, storing image data of the image scanner device into a buffer memory, stopping an image reading operation in progress, and so forth are measures that are generally taken. The former measure of storing the image data into the buffer memory and then making the transmission corresponding to the processing speed of the host device is effective and easy. However, the amount of image data to be handled has been increased in association with the improvement in resolution and the treatment of a color image and hence the provision of a buffer memory corresponding to one image results in a high cost. As a result, it cannot be said that this measure is expedient.
The latter measure is a more simple and economical measure and involves of temporarily stopping the reading operation. In a device having such a temporary stop function, in order to avoid a deviation in the reading position in the case where a rereading operation is performed after the temporary stop of a reading operation, a horizontal synchronizing signal is resynchronized so as to always keep the storage time of a line sensor (mentioned just below) constant. Thereby, double reading of an image or a miss in reading is prevented.
The line sensor is composed of a light receiving portion including a multiplicity of aligned light receiving elements for photoelectric conversion, a transfer portion including shift registers provided corresponding to the respective light receiving elements, and a gate portion provided between the light receiving portion and the transfer portion. Each light receiving element of the light receiving portion generates electric charges corresponding to a received light amount and the generated charges are stored thereinto in accordance with the lapse of time. At a point of time when the gate is opened by a shift pulse signal after the lapse of a predetermined charge storage time, the stored charges are all transported to the corresponding shift register. The charges transported to each shift register of the transfer portion are transferred in accordance with transfer clocks successively applied to the shift registers and are outputted as a serial signal from an output terminal.
FIG. 9A is a timing chart showing a shift pulse signal, FIG. 9B as a timing chart showing a transfer clock for each pixel, FIG. 9C is a timing chart showing a line sensor output signal, FIG. 9D is a timing chart showing a sample pulse, and FIG. 9E is a timing chart showing an output signal (or sample hold output signal) c of the line sensor. The line sensor output signal shown in FIG. 9C includes a reset pulse signal a, a field through signal b and the output signal c. Also, the line sensor has a sample and hold circuit and a line clamp circuit for performing a line clamp by the field through signal b. As shown in FIG. 9E, the output signal c of the line sensor has a light-shielded output signal and an effective pixel signal for each line. The light-shielded output signal is a signal indicative of a dark output level of the line sensor and is used as an offset voltage in the case where a line clamp is performed for each line. In general, the signal processing of the output signal c of the line sensor as an analog signal is such that the output signal c of the line sensor after amplified in its AC level is DC-clamped with the level of the light-shielded output signal taken as an offset level (that is, the level of the light-shielded output signal is clamped to a fixed DC level), and then A/D conversion is performed.
Next, FIGS. 10A to 10C will be used to explain a line clamp pulse defining timing at which the above-mentioned DC clamp is performed. FIG. 10A is a timing chart showing a horizontal synchronizing signal, FIG. 10B is a timing chart showing an inverted version of the output signal c of the line sensor, and FIG. 10C is a timing chart showing a line clamp pulse. As shown in FIGS. 10A to 10C, the line clamp pulse is generated at the same period as the horizontal synchronizing signal (equivalent to the shift pulse signal shown in FIG. 9A) and at a timing at which the light-shielded output signal of the line sensor is outputted.
As a responsibility at the time of charging and discharging of an offset voltage for a coupling condenser in the above-mentioned DC clamp, it is preferable that the charging and discharging are completed within the duration of one line clamp pulse. As a storage time and a transfer clock period are shortened due to the improvement of the reading speed, the completion of the response within a light-shielded output signal generation time requires a high-speed clamp buffer. However, even if the response is not completed within the light-shielded output signal generation time of the line sensor corresponding to one line, the completion of the response in a warm-up period after the turn-on of a power supply suffices to provide a function as the offset control since the potential of the coupling condenser after that is little.
This will now be explained using FIGS. 11A to 11F. FIG. 11A is a timing chart showing a horizontal synchronizing signal, FIG. 11B is a timing chart showing an output signal c of the line sensor, FIG. 11C is a timing chart showing an inverted version of the output signal c of the line sensor, FIG. 11D is a timing chart showing a line clamp pulse, FIG. 11E is a timing chart showing a response waveform of the output signal c of the line sensor in a warm-up period, and FIG. 11F is a timing chart showing the warm-up period. Even if about seven lines are taken to obtain a target DC level in the line clamp, as shown in FIG. 11E, the response in the case of a storage time of 3 msec per one line can be completed if there is a standby time of about 3.times.7=21 msec. If the response is once completed, there is little a change after that and hence a convergence into the tolerance of the target DC level is possible by the line clamp for every line.
However, when a buffer over-run is generated in the course of the reading of an image, a different situation may be encountered. This will be explained using FIGS. 12A to 12E. FIG. 12A is a timing chart showing a horizontal synchronizing signal, FIG. 12B is a timing chart showing a line clamp pulse, FIG. 12C is a timing chart showing an inversely amplified output signal of the line sensor, FIG. 12D is a timing chart showing an ideal response waveform in the line clamp, and FIG. 12E is a timing chart showing an actual response waveform in the line clamp. When the horizontal synchronizing signal is resynchronized due to the generation of a buffer over-run in the course of the reading of an image (see a resynchronizing point T1 in FIG. 12A associated with the restart of reading), the line sensor encounters the case where the period of the shift pulse is shorter than the time in which image data corresponding to one line is transferred, that is, a time equal to (the transfer time per one pixel).times.(the number of pixels). In this case, image data in the preceding time remaining untransferred is outputted in a form added to image data of the next line newly stored, as shown in FIG. 12C. As a result, there may be the case where the DC level changes greatly at the timing of DC clamping, thereby making it impossible to make a response to a normal offset not later than an effective line for rereading shown in FIG. 12E (or effective image data for rereading). For example, when the light-shielded output signal generation time is 10 .mu.sec for a clamp charging/discharging time constant of 10 .mu.sec, a response (in the case where it is regarded as a transient response of a primary system) is on the order of about 63% (=1-exp(-10 .mu.sec/10 .mu.sec)) and hence an offset level at the time of buffer over-run becomes larger than a normal level (or the coupling condenser for clamp is excessively charged) so that the level of the line sensor output signal is apparently lowered. This means that each time an image rereading operation is performed, the rendering of gradation corresponding to several lines is deteriorated so that lateral stripes are generated.
In the conventional approach for solving such a problem, the speed-up of a clamp buffer is contemplated so that the response to the DC offset level is completed within the duration of one line clamp pulse. For example, as the speed of the system is increased, in order to obtain a response equal to or higher than 99% in the case of a light-shielded output signal generation time of 10 .mu.sec, such a clamp buffer needs to have a charging/discharging time constant of 2 .mu.sec which is equal to one fifth of 10 .mu.sec. In order to ensure the charging/discharging time constant of 2 .mu.sec, the ON resistance of an analog switching is 20 .OMEGA. in the case where a 0.1 .mu.F coupling condenser is used. In the case where an ideal response as shown in FIG. 12D is not attained, the conventional approach includes a method which uses the fact that the response is nearly completed after, for example, five lines, that is, a method in which the timing of resynchronization of the horizontal synchronizing signal is set to five lines before the effective line for rereading and image data is made effective from image data read after the reading of image data corresponding to the five lines.
However, in the case where the speed of the line clamp response is increased by use of the high-speed clamp buffer, as mentioned above, there is a problem that in order to increase the speed of the line clamp charging/discharging response of the DC level, a clamp buffer involving a large amount of current flowing therethrough is needed, thereby increasing power consumption. Also, there is a problem in that the increase of noise components attendant upon variations in the power supply voltage caused the large amount of current deteriorates the S/N ratio of a read image.
In the above-mentioned method in which the horizontal synchronizing signal is resynchronized several lines before the point of time of reading restart, the adaptation of the moving speed of a carriage to a reading speed and the resynchronization of the horizontal synchronizing signal are made prior to the reading restart point of time and image data is made effective from image data read after reaching the actual reading restart point of time after several lines. Therefore, this method has a problem in that the moving amount of the carriage attendant upon the rereading operation becomes large.
Under the above circumstances, one of the requirements on the image reader associated with the resynchronization of a horizontal synchronizing signal is that even if the storage time of a line sensor output signal becomes shorter than the transfer time so that the level of the light-shielded output signal becomes abnormal, a disturbance of the image is not generated. Also, it is required that there be no need to cause a high-speed response of the clamp buffer. Further, it is required that even in the case where a rereading operation is performed, the positional deviation of the joint of images is not generated. Furthermore, it is required that the operation of a carriage until the restart of reading is minimized.